Protein kinases are important regulators of cellular proliferation and several, such as HER-2/neu, and the epidermal growth factor receptor (EGFR), have been implicated in the pathogenesis and progression of human breast cancer. We have speculated that other kinases expressed in breast cancer cells may have important functions in the genesis or maintenance of the malignant state. For this reason, we have completed a search for novel kinases in breast cancer tissue and have identified several potentially important species: Our work with the focal adhesion kinase (FAK) is among the more advanced subprojects. We have found that FAK expression is correlated with the invasive and metastatic phenotype and thus may represent a useful marker in early stage breast cancer. Moreover, specific attenuation of FAK expression by antisense oligonucleotides induces apoptosis in cancer cell lines making FAK a rational target for gene directed therapeutics. rak is a src related tyrosine kinase with a nuclear localization signal within its SH2 domain, rak is a unique nuclear tyrosine kinase that binds to the retinoblastoma gene protein, Rb, through its SH3 domain. This implicates rak in cell cycle control and potentially in transformation. STK1 is the human homolog of the cdc2 activating kinase (CAK) which is essential for cdc2/cyclin function. Our investigations show that inhibition of CAK expression results in attenuation of growth in a breast cancer cell line. TK5 is a new JAK kinase, JAK3, expressed in primary breast cancer cells and cell lines, and which is also the important kinase partner for the IL-2/IL-4 receptor. Our discovery of different splice forms with one leading to an enzymatically inactive JAK3 kinase suggests complex signalling through potentially novel cytokine receptors. In this proposal we will focus upon FAK, rak and CAK, characterizing the role of these novel species in breast cancer biology especially in growth and differentiation and to using this knowledge in the design of more specific and directed therapies for breast cancer.